Preserve last entered text

[al-develop]

Add a new setting: bool "Preserve last entry" -> displayed as a checkbox.
When it is set to true, then the content of the rich text box should be saved in a temporary file, when the application is closed.
On the next start of the application, the content of the temporary file should be loaded back into the rich text box and the file can be deleted.

[al-develop]

Keep in mind: what if there are several instances of the application open? --> Preserve only the content from the last opened window. On closing the application instances one after the other, while the setting is set to true, the temporary file should be overwritten each time until the last instance is closed